In a general method of assembling components on printed circuit boards, the component leads are mounted through holes punched in the board for that purpose and the leads are then clinched, or bent, to hold the components in place during subsequent fluxing, lead cutting and soldering operations. The leads are soldered to the boards and the excess cut off. In many cases, the cut lead ends must be resoldered. These operations require hand labor and many steps, which of course is expensive. Also, solder is lost on the discarded lead ends. Further, when a double soldering step is required, the danger of board warping, due to the high temperatures employed during soldering, is increased.
With the increased complexity of electronic devices, more components are mounted on the boards which magnifies the problems. The clinched leads on the boards interfere with each other, necessitating a stepwise compound mounting whereby part of the components are assembled, soldered and their leads cut off before mounting additional components.
Further, it is most difficult to remove faulty components which must be replaced or repaired when the leads have been clinched, without damaging the board or other adjacent components. Still further, access to both sides of the board is required.
Thus, other ways of assembling printed circuit board components without clinching leads have been sought. Among other methods, components grippers, clamps and compressible foam pads, and bags loaded with small particles, e.g., beans or plastic shot, placed over the components to hold them in place, have been proposed and used to hold electronic components in place on printed circuit boards during soldering operations.
According to another method that has been tried, a molten wax coating is applied to the bottom of the board after the component leads have been inserted into the holes. After the wax cools and hardens, it holds the components in place while the leads are cut and soldered. While this method is adaptable to automatic soldering operations and pre-cutting of leads, the wax smokes during soldering operations and requires organic solvents or hot water in excess of 150.degree. F. (65.56.degree. C.) to effect its removal. In addition, a second flux over the wax coating is required, in some instances, for automatic soldering operations.
Application Ser. Nos. 771,680, filed Feb. 24, 1977; 771,679, filed Feb. 24, 1977; and 834,279, filed Sept. 19, 1977, U.S. Pat. Nos. 4,115,157; 4,113,524; and 4,143,005, respectively, assigned to the same assignee, solved the above problems through the use of extrudable, non-flowable, water or solvent soluble compositions that can be readily applied over the components assembled on the printed circuit boards to hold the components in place without lead clinching during the solder operation. However, the extrudable compositions do not permit the correction of improperly inserted components into the printed circuit board until after the completion of soldering operations.
Thus, it would be desirable to provide a method of assembling components on printed circuit boards that will permit corrections to be made in the circuit board wiring before soldering operations and maintain the unclinched components in place during soldering operations.